hvc_console: Change an mb() to smp_mb() and add some comments

I remember some history on this barrier.  There was a race between
open via /dev/console and the tty being fully setup.  Its also why
there is a temporary variable and the global is assigned at the end
of the function.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>